Fiber reinforced plastic bolt and method for producing the same

ABSTRACT

A fiber reinforced plastic bolt (FRP) and a method for producing the same are disclosed. The FRP bolt is useful in the industrial areas which require a bolt having electric-insulation, anti-corrosive, thermal-insulation, and non-magnetic properties. The method for producing the FRP bolt includes the steps of: winding a prepreg around a surface of an internal body, wherein the internal body includes a first reinforcing fiber which is unidirectionally aligned along the axis of the bolt, and a synthetic resin which is impregnated into the first reinforcing fiber, and the prepreg includes a second reinforcing fiber and a thermosetting resin which is impregnated into the second reinforcing fiber; forming a fiber reinforced plastic round bar by heat-hardening the prepreg; and forming a screw thread on the surface of the fiber reinforced plastic round bar.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending application Ser. No.13,321,414 filed on Nov. 18, 2011, application Ser. No. 13/321,414 is aNational Phase of PCT International Application No. PCT/KR2010/003564filed on Jun. 3, 2010, which claims priority under 35 U.S.C. 119(a) toPatent Application No. 10-2009-0048879 filed in the Republic of Korea onJun. 3, 2009, all of which are hereby expressly incorporated byreference into the present application.

TECHNICAL FIELD

This invention relates to a fiber reinforced plastic bolt and a methodfor producing the same. The fiber reinforced plastic bolt isparticularly useful in the industrial areas which require a bolt havingelectric-insulation, anti-corrosive, thermal-insulation, andnon-magnetic properties.

BACKGROUND ART

Fiber reinforced plastic (FRP) is prepared by impregnating syntheticresin such as epoxy resin, vinylester resin, phenol resin, and so oninto reinforcing fiber such as glass fiber, carbon fiber, aramid fiberand so on. The fiber reinforced plastic is generally used in variousareas which require electric-insulation, thermal-insulation, andnon-magnetic properties, and is also used as a structural reinforcingmaterial in civil engineering and construction industry. The fiberreinforced plastic is also used to manufacture an article such as a boltor a nut for fixing substances.

FIGS. 3 and 4 are drawings for illustrating a method for producing a FRPbolt in accordance with a conventional method. As shown in FIG. 3, inthe conventional method, reinforcing long-fiber 12 is impregnated withliquid-phase synthetic resin, and is pultruded with a thermoforming die20 while being transported in parallel. The pultruded and syntheticresin impregnated fiber is hardened to form a FRP round bar 14. Thehardened round bar 14 is cut with a cutter 22 to a predetermined length.As shown in drawing A in FIG. 4, the surface of the FRP round bar 14 isground with a bite 24 of a screw cutting device to form a screw threadof a FRP bolt 16. As shown in drawing B in FIG. 4 which is alongitudinal sectional view of the produced FRP bolt 16, the producedFRP bolt 16 has long-fiber 12 which is unidirectionally aligned alongthe axis of the bolt 16. Thus, the screw thread of the FRP bolt 16 iseasily damaged by an external force, and the FRP bolt 16 is not suitablefor connecting structures or substances in which a strong force isapplied. In other words, as the surface of the FRP round bar 14 isground away to form the screw thread of the FRP bolt 16, the reinforcinglong-fiber 12 (filament) in the screw thread is cut off, the mechanicalstrength of the long-fiber 12 is deteriorated, and the tensile strengthof the FRP bolt 16 is also deteriorated.

Meanwhile, in order to increase the strength of screw thread of a FRPbolt, Japanese Patent Laid-Open No. H07-279933 discloses a bolt havingan internal body and a screw thread which are made of differentmaterials from each other, wherein the screw thread covers the internalbody and is produced by a molding process. However, the resinimpregnation process and the hardening process in a mold are complicatedin the method, and thus the productivity of the bolt is not desirable.Alternatively, Japanese Patent Laid-Open No. H06-114859 discloses amethod for producing a bolt including the step of forming a round barmade of reinforcing fiber impregnated with thermoplastic resin with thefirst mold, and forming the round bar into a bolt having a screw threadwith the second mold. However, this method requires a molded preformmade of thermoplastic resin, and the preform should be molded again.Thus, the method is not desirable in its productivity. Furthermore, forproducing a screw thread of a FRP bolt by a molding process, severalmolds should be prepared for each type or size of the bolt, which resultin the high production cost and the low productivity.

DISCLOSURE Technical Problem

Therefore, it is an object of the present invention to provide a fiberreinforced plastic bolt having a screw thread of high strength. It isother object of the present invention to provide a method for producinga fiber reinforced plastic bolt having a superior quality with adesirable productivity.

Technical Solution

In order to achieve these and other objects, the present inventionprovides a method for producing a fiber reinforced plastic boltcomprising the steps of: winding a prepreg around a surface of aninternal body, wherein the internal body includes the first reinforcingfiber which is unidirectionally aligned along the axis of the bolt, anda synthetic resin which is impregnated into the first reinforcing fiber,and the prepreg includes the second reinforcing fiber and athermosetting resin which is impregnated into the second reinforcingfiber, forming a fiber reinforced plastic round bar by heat-hardeningthe prepreg; and forming a screw thread on the surface of the fiberreinforced plastic round bar. The present invention also provides afiber reinforced plastic bolt comprising: an internal body whichincludes the first reinforcing fiber unidirectionally aligned along theaxis of the bolt, and a synthetic resin which is impregnated into thefirst reinforcing fiber, and a hardened prepreg layer which is woundaround the internal body, and includes the second reinforcing fiber anda thermosetting resin which is impregnated into the second reinforcingfiber, wherein a screw thread is formed on an outer surface of thehardened prepreg layer.

Advantageous Effects

In the fiber reinforced plastic (FRP) bolt of the present invention, ascrew thread is formed on the hardened prepreg layer having the secondreinforcing fiber. The hardened prepreg layer is formed by winding theprepreg around the internal body at least one turn. Therefore, even ifthe second reinforcing fiber is partially cut or damaged by forming thescrew thread, the second reinforcing fiber remains intact and isdirected in a circular direction of the screw thread. The remainingreinforcing fiber increases the strength of the screw thread.Furthermore, in the FRP bolt of the present invention, the internal bodyand the hardened prepreg layer firmly bind together, which increases thetensile strength of the FRP bolt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing for illustrating a method for producing a FRP boltaccording to an embodiment of the present invention.

FIG. 2 shows a longitudinal sectional view (A) and a cross sectionalview (B) of a FRP bolt according to an embodiment of the presentinvention.

FIGS. 3 and 4 are drawings for illustrating a method for producing a FRPbolt according to a conventional method.

DETAILED DESCRIPTION OF THE INVENTION

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be better appreciated by reference to thefollowing detailed description.

FIG. 1 is a drawing for illustrating a method for producing a FRP boltaccording to an embodiment of the present invention. As shown in FIG. 1,the method for producing a FRP bolt according to an embodiment of thepresent invention includes the step of winding a prepreg 44 around thesurface of an internal body 34 (See drawing A of FIG. 1). The internalbody 34 includes the first reinforcing fiber 32 which isunidirectionally aligned along the axis of the bolt, and a syntheticresin which is impregnated into the first reinforcing fiber 32. Theprepreg 44 includes the second reinforcing fiber 42 and a thermosettingresin which is impregnated into the second reinforcing fiber 42.Preferably, as shown in FIG. 1, the internal body 34 has the shape of around bar, and the prepreg 44 has the shape of a sheet. The winding ofthe prepreg 44 around the internal body 34 can be carried out at least 1turn, preferably 2˜50 turns, more preferably 10˜40 turns so that a screwthread can be formed on the wound prepreg layer. Then, the wound prepreg44 is heat-hardened to form a fiber reinforced plastic round bar 54.Preferably, the heat-hardening can be carried out by winding a heatshrinkable film 52 around the surface of the wound prepreg 44 andheating the heat shrinkable film 52 and the prepreg 44 (See drawing B ofFIG. 1). By the heat-hardening, the internal body 34 and the layer ofthe prepreg 44 firmly bind together. Next, the screw thread is formed onthe surface of the fiber reinforced plastic round bar 54 with a cuttingdevice 62 such as a bite to produce the FRP bolt 64 of the presentinvention (See drawing C of FIG. 1).

In the internal body 34, the first reinforcing fiber 32 is used toreinforce the synthetic resin, and examples of the first reinforcingfiber 32 include carbon fiber, glass fiber, aramid fiber, mixturesthereof, and so on. The selection of the first reinforcing fiber 32depends on the use or the required properties of the produced FRP bolt64. The internal body 34 mainly increases the bending resistance of theFRP bolt 64. Thus, the preferable first reinforcing fiber 32 is longfiber rather than short staple or monofilament, and is unidirectionallyaligned along the direction of the internal body 34 and the FRP bolt 64.If the short staple or monofilament is used as the first reinforcingfiber 32, the mechanical strength of the FRP bolt 64 may not besatisfactory. Thus, the more preferable first reinforcing fiber 32 islong fiber continuously extended along the direction of the internalbody 34. Examples of the synthetic resin for impregnating the firstreinforcing fiber 32 include epoxy resin, polyester resin, vinylesterresin, phenol resin, mixtures thereof, and so on. In the internal body34, the volume amount (Vf) of the first reinforcing fiber 32 ispreferably 10˜90%, more preferably 30˜80%, and most preferably 45˜60%based on the total volume of the internal body 34. If the volume amount(VT) of the first reinforcing fiber 32 is less than the above-mentionedrange, the bending resistance of the FRP bolt 64 may be unsatisfactory.If the volume amount (VT) of the first reinforcing fiber 32 is more thanthe above-mentioned range, the internal body 34 may not be properlyformed and void can be produced between the first reinforcing fiber 32,which deteriorate the mechanical strength of the internal body 34. Theinternal body 34 can be produced by pultruding the first reinforcingfiber 32 and the synthetic resin. For example, the internal body 34 hasa shape of a round bar or a circular cylinder having a diameter of 1˜50mm, preferably 3˜20 mm. If necessary, before winding the prepreg 44, thesurface of the internal body 34 can be polished and washed.

The second reinforcing fiber 42 used in the prepreg 44 can also be madeof carbon fiber, glass fiber, aramid fiber, and mixtures thereof. Theshape of the second reinforcing fiber 42 can be selected from the groupconsisting of fabric (plain fabric, plain weave, satin weave, etc) whichis woven with long fiber, unidirectionally aligned long fiber,multidirectionally, i.e., randomly aligned short staple or monofilamentsheet, non-woven fabric of short staple or monofilament, mixturesthereof, and so on. In this case, the length of the short staple or themonofilament is generally 1˜100 mm, and preferably 2˜50 mm. Consideringthe workability and the productivity for producing the FRP bolt 64, themore preferable second reinforcing fiber 42 has the shape of fabric ornon-woven fabric. When a plain fabric shaped reinforcing fiber 42 isused as the second reinforcing fiber 42, the angle between the directionof the internal body 34 and the fiber in the plain fabric shapedreinforcing fiber 42 is between 0˜90 degree, for example, between 10˜80degree, more specifically between 30˜60 degree. For example, as shown inFIG. 1, if the long fibers of the fabric shaped reinforcing fiber 42which direct to a horizontal direction and a vertical direction (i.e.,perpendicularly, with 90 degree) are wound in parallel around theinternal body 34, the angles between the internal body 34 and fibers inthe reinforcing fiber 42 are 0 degree and 90 degree. Meanwhile, if thelong fibers in the fabric shaped reinforcing fiber 42 which direct to ahorizontal direction and a vertical direction (i.e., perpendicularly,with 90 degree) are wound around the internal body 34 with the angle of45 degree, the angles between the internal body 34 and the fibers in thereinforcing fiber 42 are 45 degree and −45 degree. For increasing thestrength of the FRP bolt 64, the angles are preferably 0 degree and 90degree.

As the thermosetting resin which is impregnated into the secondreinforcing fiber 42 in the prepreg 44, the synthetic resin useful forthe internal body 34 can also be used. Specific examples of thethermosetting resin include epoxy resin, polyester resin, vinylesterresin, phenol resin, mixtures thereof, and so on. It is preferable thatthe thermosetting resin can be hardened within 120 minutes at thetemperature of 80˜250° C. As the resins for the prepreg 44 and theinternal body 34, different resins can be used. However, if same kind ofresin is used for the prepreg 44 and the internal body 34, the bindingstrength between the internal body 34 and the prepreg 44 may increase.

In the thermosetting prepreg 44, the volume amount (Vf) of the secondreinforcing fiber 42 is preferably 10˜90%, more preferably 30˜80%, andmost preferably 45˜60% (in this case, the volume amount (Vf) of theresin is 40˜55%) based on the total volume of the prepreg 44. If thevolume amount (Vf) of the second reinforcing fiber 42 is less than theabove-mentioned range, the strength of the screw thread may beinsufficient. If the volume amount (Vf) of the second reinforcing fiber42 is more than the above-mentioned range, the amount of resin in theprepreg 44 is insufficient, and the winding of the prepreg 44 on theinternal body 34 cannot be properly performed, and the quality of theFRP bolt 64 may be deteriorated. In the thermosetting prepreg 44, theweight of the second reinforcing fiber 42 is preferably 5˜500 g/m².Preferably, the prepreg 44 has the shape of a sheet, and the preferablethickness of the sheet is 0.02˜2.5 mm. The number of turns of theprepreg 44 wound around the internal body 34 can be determined accordingto the depth of the screw thread, and the preferable number of the turnsis 2˜50.

For heat-hardening the prepreg 44 which is wound on the internal body34, the internal body 34 and the prepreg 44 can be inserted into a moldhaving a smaller diameter than the diameter of the prepreg 44, and heatand pressure can be applied to the prepreg 44 in the mold for theheat-hardening. However, in this case, several molds should be preparedfor each type, size or diameter of the prepreg 44, which results in thehigh production cost and the low productivity. Therefore, in the presentinvention, a heat shrinkable film 52 is preferably used for theheat-hardening process. Specifically, the heat shrinkable film 52 iswound around the prepreg 44, and heat and pressure are applied to theheat shrinkable film 52, the prepreg 44 and the internal body 34, forexample, in an oven, to produce the fiber reinforced plastic round bar54. The conventional heat shrinkable film can be used in the presentinvention. The heat shrinkable film 52 can be made of polypropylene,polyester, and so on, and the shrinkage ratio of the heat shrinkablefilm 52 can be controlled so that the prepreg 44 is heat-hardened tohave a predetermined size according to the properties of the prepreg 44and the internal body 34. The prepreg 44 has been conventionally used inproducing a round bar such as a fishing pole, a golf shaft, and so on,and is suitable for producing the round bar in large quantity.

Next, the screw thread is formed on the surface of the fiber reinforcedplastic round bar 54, more specifically, on the hardened prepreg 44 toproduce the FRP bolt 64. The screw thread can be formed with aconventional cutting device 62 such as a bite for grinding a screwthread. In forming the screw thread, it is preferable that the root ofthe screw thread is formed in the hardened prepreg layer 44, rather thanin the internal body 34. More preferably, the distance between the rootof the screw thread and the internal body 34, in other words, theminimum thickness of the non-grinded and remaining prepreg 44 is morethan 0.1 mm, more preferably 0.5 mm, specifically 0.5˜5 mm. If thedistance between the root of the screw thread and the internal body 34is less than 0.1 mm, the prepreg 44 forming the screw thread can beseparated from the internal body 34, and the tensile strength of thescrew thread may decrease.

FIG. 2 shows a longitudinal sectional view (A) and a cross sectionalview (B) of a FRP bolt according to an embodiment of the presentinvention. As shown in FIG. 2, the FRP bolt 64 of the present inventionhas a multi-layer structure, which comprises the internal body 34 whichincludes the first reinforcing fiber 32 unidirectionally aligned alongthe axis of the bolt, and a synthetic resin which is impregnated intothe first reinforcing fiber 32, and the hardened prepreg layer 44 whichis wound around the internal body 32, and includes the secondreinforcing fiber 42 and a thermosetting resin which is impregnated intothe second reinforcing fiber 42, wherein a screw thread is formed on anouter surface of the hardened prepreg layer 44. If desirable, the heatshrinkable film 52 is wound around the prepreg 44 where the screw threadis not formed as shown in drawing C of FIG. 1. In the FRP bolt of thepresent invention, the screw thread is formed on the prepreg 44, and theprepreg 44 firmly binds with the internal body 34, which increase thestrength of the screw thread.

Hereinafter, the preferable examples and comparative examples areprovided for better understanding of the present invention. However, thepresent invention is not limited by the following examples.

EXAMPLE 1 Preparation of FRP Bolt

A round bar (internal body) having the diameter of 8 mm which includedglass fiber unidirectionally aligned along the bar's axis and epoxyresin impregnated into the glass fiber was prepared by a pultrusionmethod. A fabric prepreg having the thickness of 0.13 mm which includedplain fabric structured glass fiber and epoxy resin impregnated into theglass fiber was wound around the round bar (internal body). The anglesbetween the fiber of the fabric prepreg and the axis of the round barwere 0 degree and 90 degree. Then, a polypropylene heat shrinkable filmhaving the width of 15 mm and the thickness of 30 micron was woundaround the prepreg with a tensile force of 3 kgf and a pitch of 1.5 mm.The wound prepreg was hardened for 80 minutes at 125° C. to produce aFRP round bar having the diameter of 13 mm. The hardened prepreg wasgrinded with a cutting device to form a screw thread (pitch: 1.5, M12).The depth of the screw thread was controlled so that the distancebetween the root of the screw thread and the internal body is 1.2 mm.

EXAMPLE 2 Preparation of FRP Bolt

A round bar (internal body) having the diameter of 8 mm which includedglass fiber unidirectionally aligned along the bars axis and epoxy resinimpregnated into the glass fiber was prepared by a pultrusion method. Afabric prepreg having the thickness of 0.13 mm which included plainfabric structured glass fiber and epoxy resin impregnated into the glassfiber was wound around the round bar (internal body). The angles betweenthe fiber of the fabric prepreg and the axis of the round bar were 45degree and −45 degree. Then, a polypropylene heat shrinkable film havingthe width of 15 mm and the thickness of 30 micron was wound around theprepreg with a tensile force of 3 kgf and a pitch of 1.5 mm. The woundprepreg was hardened for 60 minutes at 125° C. to produce a FRP roundbar having the diameter of 13 mm. The hardened prepreg was grinded witha cutting device to form a screw thread (pitch: 1.5, M12). The depth ofthe screw thread was controlled so that the distance between the root ofthe screw thread and the internal body is 1.2 mm.

COMPARATIVE EXAMPLE Preparation of FRP Bolt

A FRP round bar having the diameter of 13 mm which included glass fiberunidirectionally aligned along the bar's axis and epoxy resinimpregnated into the glass fiber was prepared by a pultrusion method.The FRP round bar was grinded with a cutting device to form a screwthread. The screw thread was formed to have same size with the FRP boltsof Examples 1 and 2 (pitch: 1.5, M12).

For measuring the tensile strength of the FRP bolt produced in Examplesand Comparative example, two steel nuts having the height of 9.8 mm wererespectively fixed at each end of the FRP bolt having the length of 160mm. Tensile strength was applied to the FRP bolt with a speed of 2.54mm/minute in a material testing machine, and the tensile strengthendured by the screw thread of the FRP bolt was measured and is setforth in Table 1.

TABLE 1 Tensile strength (kgf) Improve ratio of tensile strengthComparative 200 100% example Example 1 640 320% Example 2 500 250%

As shown in Table 1, when a screw thread is formed on the prepreg layer,the strength of the screw thread improves and the tensile strength ofthe FRP bolt remarkably increases. In addition, when the directions ofthe reinforcing fiber in the prepreg are 0 degree and 90 degree (Example1), the tensile strength of the FRP bolt is superior to the FRP bolt ofExample 2 in which the directions of the reinforcing fiber are 45 degreeand −45 degree. Furthermore, when the directions of the reinforcingfiber of the FRP bolt are 0 degree and 90 degree (Example 1), thecutting process of the prepreg can be more easily carried out.

This application claims the priority benefit of Korean PatentApplication No. 10-2009-0048879 filed on Jun. 3, 2009. AU disclosure ofthe Korean Patent application is incorporated herein by reference.

The invention claimed is:
 1. A fiber reinforced plastic bolt comprising:an internal body which includes a first reinforcing fiberunidirectionally aligned along the axis of the bolt, and a syntheticresin which is impregnated into the first reinforcing fiber, and ahardened prepreg layer which is wound around the internal body, andincludes a second reinforcing fiber and a thermosetting resin which isimpregnated into the second reinforcing fiber, wherein the hardenedprepreg layer is formed by winding a prepreg having a shape of a sheetand a thickness of 0.02 to 2.5 mm around the internal body 2 to 50turns, and a screw thread is formed on an outer surface of the hardenedprepreg layer, and wherein the volume amount of the second reinforcingfiber is 45 to 60% and the volume of the thermosetting resin is 40 to55% based on the total volume of the prepreg.
 2. The fiber reinforcedplastic bolt of claim 1, wherein the first reinforcing fiber is longfiber continuously extended along the direction of the internal body. 3.The fiber reinforced plastic bolt of claim 1, wherein a distance betweena root of the screw thread and the internal body is more than 0.1 mm.